Spread frequency-shift keying (S-FSK) is a modulation and demodulation technique that combines advantages of a classical spread spectrum system (e.g., immunity against narrowband interferences) with advantages of a classical FSK system (e.g., low-complexity). An S-FSK transmitter outputs a tone at one of two frequencies depending on the value of a digital data bit. The frequencies may be referred to as a “mark” frequency (fM) and a “space” frequency (fS) (see FIG. 14). For example, the S-FSK transmitter may transmit a signal on the “space” frequency to represent an “OFF” data bit and on the “mark” frequency to represent an “ON” data bit. The difference between S-FSK and classical FSK is that the fM and fS frequencies are farther apart from each other (“spread”). By placing fS far from fM, the channel effect on the quality of the received two signals becomes independent. In other words, each frequency will have its own attenuation factor and local narrow-band noise spectrum. Thus, a narrow band interferer only affects one of the two frequency signals.
An S-FSK receiver performs FSK demodulation at the transmitted “mark” and “space” frequencies resulting in two demodulated signals, fM for the “mark” frequency and fS for the “space” frequency (see FIG. 14). If the average reception quality of the demodulated “mark” and “space” frequency signals is similar, a decision unit may decide the value of the digital data bit based on the demodulated signal with the higher reception quality. If, however, the average reception quality of one demodulated frequency signal is better than the quality of the other frequency signal, the decision unit may compare the demodulated signal of the better channel with a threshold (T) in deciding the value of the digital data bit. In other words, the S-FSK receiver could perform an FSK demodulation if both channels are good or an on-off keyed (OOK) demodulation if one channel is bad. In this scenario, the decision unit ignores the demodulated signal having lower quality. Depending on the application for S-FSK modulation, there could be periods of zero energy in the transmitted frequency signals. If the average reception quality is below the threshold (T) for both demodulated frequencies, the decision unit may interpret this condition as a zero-energy state. Higher level coding may be employed in the S-FSK transmitter to generate bit-streams that represent code words or commands which are modulated in the S-FSK waveform.
For example, SunSpec Interoperability Specification, Communication Signal for Rapid Shutdown, Version 34, describes an S-FSK communication system for transmission and reception of S-FSK waveforms carrying Barker codes representing a sequence of “ON” and “OFF” digital data bits that are modulated and demodulated based on the “mark” and “space” frequencies of the S-FSK modulation scheme. This S-FSK communication system uses power line communication (PLC) techniques to exchange sequences of Barker code words that represent commands for controlling photovoltaic (PV) arrays. For example, commands can be used to implement rapid shutdown or other commands can be used to keep the arrays alive. FIG. 15 shows PLC physical layer transmission format requirements presented in SunSpec Interoperability Specification, Communication Signal for Rapid Shutdown, Version 34.